In those motor vehicle air conditioning and heating systems where the hot liquid flow from the engine's coolant system through the heater core is controlled to control the temperature of the air delivered to the passenger compartment, conventional practice has been to pass all the air through both the evaporator and heater core with the tempered air then distributed to the head and foot regions of the passenger compartment at the same temperature. As a result, there is no so-called bi-level air distribution within the passenger compartment in the air conditioning mode. Such bi-level air distribution is desirable since for example on a winter day with a high sun load via the windows, it may be more comfortable to the passengers to have the air delivered to the foot or floor region warmer than that delivered to the head region. And likewise on a summer day it may be more comfortable to have the air delivered at head level cooler than that delivered at foot level.